Device for detecting the amount of splash water to which a brake pad is subjected on a wet roadway

ABSTRACT

A device for detecting the amount of splash water to which a brake pad is subjected on a wet roadway. The device includes a test brake pad, whose friction lining is hygroscopically designed so that it can absorb at least 5%, preferably at least 10% water. In a test phase, vehicles can be easily tested by replacing their stock brake pads with test brake pads. By virtue of the fact that the inventive friction lining is hygroscopic, a direct correlation exists between the water absorption of the friction lining and the amount of moisture led to the friction lining in the form of splash water and air humidity.

The present invention relates to a device for determining the amount ofsplash water, to which a brake pad is subjected on a wet roadway and toa usage of the device.

It is important for brake pads that they exhibit stable frictionalvalues, even in wet conditions. Water in the form of spray waterbasically has a negative effect on the frictional values of a brake pad.It has transpired that with unfavourable constructions of the vehicleand/or brakes, the braking distance of a vehicle in winter can increaseby a factor of up to 2 under the influence not only of water but also ofroad salt. This is of crucial significance for the safety of traffic.Furthermore, the comfort performance and the corrosion of the vehicleare negatively influenced by an unnecessarily high supply of sprayedwater.

It is therefore the object of the present invention to provide a deviceof the type referred to above and a usage for this device.

In accordance with the invention, this object is solved by a device fordetermining the amount of splash water, to which a brake pad issubjected on a wet roadway, including a test brake pad, whose frictionlining is hygroscopic such that it can absorb at least 5%, preferably atleast 10%, water.

Substances which can withdraw the water vapour from a water vapour-gasmixture, eg. air, are termed hygroscopic. Hygroscopic substances alsoabsorb water when in direct contact with water, for instance when incontact with splash water in the invention. The water can be absorbedeither by hydration, wherein water molecules are accumulated at polargroups of molecules dissolved in the water on the basis of electrostaticion-dipole interaction or by forming hydrogen bridge compounds. Theabsorbed water can be gently removed again by the supply of energy. Thisis referred to as drying. The process of water absorption and dryingoccurs in certain temperature ranges without molecular conversion orsplitting and is thus reversible in these ranges. The test brake pads inaccordance with the invention can thus be used repeatedly, when usingsuitable test conditions, if they are dried sufficiently in themeantime. The material costs can be limited to a minimum in this manner.

Surprisingly, it has transpired that complicated test structures fordetermining the flow conditions of the spray water are not necessary.The vehicles can be tested in a test phase simply by replacing theirstandard brake pads by test brake pads. Since the friction lining inaccordance with the invention is hygroscopic, there is a directconnection between the water absorption of the friction lining and theamount of moisture supplied to the friction lining in the form of spraywater and air humidity. The test brake pad in accordance with theinvention thus permits a precise determination of the moisture suppliedto the brakes. The amount of spray water, to which a brake pad issubjected on a wet roadway, may be determined from this suppliedmoisture, either with the aid of comparative measurements or by means ofa computer. In the simplest case, the influence of the air humidity canbe ignored. In this manner, the information necessary for optimisationof the vehicle construction, particularly of the underbody assemblyand/or of the brakes as regards the supply of spray water can beobtained without particular effort and thus economically.

The above object is also solved in accordance with the invention by ause of the device in accordance with the invention, wherein

-   -   a) the initial mass of the test brake pad is determined,    -   b) the test brake pad is installed in a vehicle,    -   c) the vehicle is subjected to predetermined operational        conditions,    -   d) the final mass of the test brake pad is then determined and    -   e) the water absorption of the friction lining is determined        from the difference between the initial mass and the final mass.

Particularly high precision in the determination of the water absorptioncan be achieved if the test brake pad is dried before step d). Wateradhering to the surface of the test brake pad may thus be removed sothat it does not enter into the determination of the final mass of thetest brake pad. The drying is effected such that the water absorbed bythe test brake pad is not expelled. There is thus a brief dryingprocess. It is particularly advantageous to wash off the test brake padwith a low boiling point liquid. Acetone or ethanol, above all, aresuitable for this purpose. These liquids remove the surface water andthe subsequent drying process then serves only to vaporise the washingliquid. The energy input necessary for this purpose is too low to expelthe water absorbed by the test brake pad.

The steps a) to e) are advantageously performed for all brake pads of avehicle. Problems with splash water generally occur more at the innerbrake pads of vehicles than at the outer pads. As a result of thedetermination of the water absorption of all brake pads of a vehicle,the vehicle construction or the brake construction can be optimisedsubstantially better, more rapidly and more economically.

A preferred exemplary embodiment is characterised in that a thresholdvalue is determined for the water absorption and the construction of thevehicle and/or brakes is altered when the water absorption determined instep e) is greater then the threshold value.

The test brake pad in accordance with the invention is advantageouslycharacterised in that the friction lining contains at least 15 vol. %,preferably at least 17 vol. %, hygroscopic bonding agent. Thehygroscopic property of the friction lining can be achieved withoutdifficulty by means of a substantially higher content of bonding agentsin comparison to the prior art without special additives being requiredor mechanical treatment of the friction lining being necessary toachieve a porous body. The bonding agent can be selected at will independence on the desired manufacturing process, for instance with orwithout scorching. Resins (unmodified or organically or inorganicallymodified) are generally used as the bonding agent. Of the organicmodifications, the cresol, alkyl, CSNL, epoxide, NBR, resorcinol, aryl,SBR and CR modifications have proved to be particularly satisfactory andof the inorganic modifications, the boron, phosphorus, silicone andchromium modifications have proved to be particularly satisfactory.

It has proved to be satisfactory if the friction lining is free oflubricant and, in particular, contains no sulphides or graphite. Oneembodiment of the invention is characterised in that the friction liningis additionally or alternatively free of abrasive agent and, inparticular, contains no Al₂O₃, no Zr silicate and no SiC.

The term abrasive agents is to be understood in this case as abrasivefrictional particles in the classical sense. It has transpired that afriction lining free of lubricant and free of abrasive agent isparticularly suitable for testing the vehicle construction as regardsthe supply of splash water since it can not only be adjusted to beappropriately hygroscopic but also exhibits excellent frictionalcharacteristics.

It is proposed in a further embodiment of the invention that thefriction lining contains 8 vol. % to 12 vol. %, preferably 10 vol. %fibres.

The friction lining preferably contains aramide fibres and/orpolyacrylonitrile fibres as the fibres. Aramide fibres, which alreadycontain up to 5% water, can be used in order to reduce the electrostaticcharge. These are able, under ideal conditions, permanently to storemore than 15% water.

A preferred exemplary embodiment is characterised in that the frictionlining contains

6 vol. % to 14 vol. %, preferably 10 vol. %, fibres,

5 vol. % to 13 vol. %, preferably 9 vol. %, rubber,

13 vol. % to 21 vol. %, preferably 17 vol. %, bonding agent,

10 vol. % to 18 vol. %, preferably 14 vol. %, amorphous quartz,

1 vol. % to 9.5 vol. %, preferably 5.5 vol. %, mica,

10.5 vol. % to 18.5 vol. %, preferable 14.5 vol. %, magnesium-aluminiumsilicate,

5.5 vol. % to 13.5 vol. %, preferably 9.5 vol %, potassium titanate,

6.5 vol. % to 14.5 vol. %, preferably 10.5 vol. %, steel wool, and

6 vol. % to 14 vol. %, preferably 10 vol. %, aluminium hydrosilicate

Both expanded and non-expanded mica can be used as the mica. Potassiumtitanate can be included in the firm of fibres or in powder form. Thesteel wool is for the purpose of mechanical strength. It is inherentlynon-hygroscopic.

For test purposes, an appropriate friction lining was stored at 95% airhumidity. It transpired that the material absorbed 3.4% water after only2 days, 5.8% after 5 days, 10.2% after 14 days, 13.4% after 35 days and15% after 49 days. This slow, continuous increase in the waterabsorption is optimal for detecting the amount of spray water to which abrake pad is subjected on a wet roadway, since the friction pad can notimmediately achieve its maximum water absorption in the test phase, whenit comes into contact with water. On the other hand, the waterabsorption should not occur too slowly since the test phase should notexceed reasonable time limits. This is possible without difficulty withthe aforementioned composition. Furthermore, a test brake pad with afriction pad of this composition exhibits stable frictional values ofbetween μ=0.3 and 0.4 under frictional testing. The cold frictionalvalue of μ=0.4 is also very favourable and higher than with many massproduced materials. Even emergency braking from 100 and 180 km/h isstable. It is notable that such a test brake pad also has astonishinglyshort moment curves. Only at the first temperature fading, namely whenthe final temperature rises to 600° C., does the frictional value dropsignificantly. Overall, a test brake pad with a friction pad of theaforementioned composition has frictional characteristics comparable tothe known, less hygroscopic, mass produced materials under temperatureloading of up to ca. 400° C. Under the braking procedures performed inthe test phase, account need consequently not be taken of the particularmaterial characteristics of the test brake pad which is used. Theoperational conditions, to which the vehicle is subjected in the testphase, can therefore be selected to be as realistic as possible.

A further advantage of the aforementioned composition is that acorresponding test brake pad results in no perceptible noise and, inparticular, has a very good squeaking behaviour.

A hygroscopic rubber is preferably used as the rubber for reasons ofcompressibility. The friction pad preferably containsacrylonitrile-butadiene rubber as the rubber.

The automobile manufacturers and/or brake manufacturers can developremedial measures to reduce the supply of splash water on the basis ofthe amount of splash water, to which a brake pad is subjected on a wetroadway and which may be detected with the test brake pad in accordancewith the invention. They can alter the construction of the vehicle or ofthe brakes or incorporate additional elements, for instance protectiveshields or deflecting spoilers. The braking distance can be shortenedunder wet conditions in this manner and the safety of the vehicle andthus of the traffic can be significantly increased. At the same time,corrosion is reduced. Numerous developments are possible within thescope of the inventive concept. The friction lining can be as stronglyhygroscopic as desired. The composition of the friction pad can also bealtered as desired, provided that a minimum amount of frictionalproperties remains. Instead of determining the initial mass and thefinal mass of the test brake pad, the corresponding initial volume andfinal volume can also be determined.

1-12. (canceled)
 13. A device for detecting the amount of splash water,to which a brake pad is subjected on a wet roadway, said devicecomprising a test brake pad, said test brake pad having a hygroscopicfriction lining which is able to absorb at least 5% water.
 14. Thedevice of claim 13, wherein the friction lining can absorb at least 10%water.
 15. The device of claim 13, wherein said friction lining containsat least 15 vol. % of a hygroscopic bonding agent.
 16. The device ofclaim 13, wherein said friction lining is free of lubricant and containsno sulphides or graphites.
 17. The device of claim 17, wherein saidfriction lining is free of abrasive agents.
 18. The device of claim 17,wherein said friction lining contains no Al₂O₃, no Zr silicate and noSiC.
 19. The device of claim 13, wherein said friction lining contains 8vol. % to 12 vol. % fibres.
 20. The device of claim 19, wherein saidfriction lining contains 10 vol. % fibres.
 21. The device of claim 19,wherein said friction lining contains aramide fibres and/orpolyacrylonitrile fibres.
 22. The device of claim 13, wherein saidfriction lining contains 6 vol. % to 14 vol. % fibres, 5 vol. % to 13vol. % rubber, 13 vol. % to 21 vol. % bonding agent, 10 vol. % to 18vol. % amorphous quartz, 1 vol. % to 9.5 vol. % mica, 10.5 vol. % to18.5 vol. % magnesium-aluminium silicate, 5.5 vol. % to 13.5 vol. %potassium titanate, 6.5 vol. % to 14.5 vol. % steel wool, and 6 vol. %to 14 vol. % aluminium hydrosilicate.
 23. The device of claim 22,wherein said friction lining contains acrylo nitrile-butadiene rubber.24. A method for determining the amount of splash water, to which abrake pad is subjected on a wet roadway, by means of a device comprisinga test brake pad, said test brake pad having a hygroscopic frictionlining which is able to absorb at least 5% water, said methodcomprising: a) determining an initial mass of the test barke pad, b)installing the test brake pad into a vehicle, c) subjecting the vehicleto predetermined operating conditions, d) determining the final mass ofthe test brake pad and e) determining the water absorption of thefriction lining from the difference between the initial mass and thefinal mass.
 25. The method of claim 24, wherein the test brake pad isdried before step d).
 26. The method of claim 24, wherein steps a) to e)are performed for all the brake pads of a vehicle.
 27. The method ofclaim 24, wherein a threshold value is established for the waterabsorption and the construction of the vehicles and/or brakes is alteredwhen the water absorption in step e) is greater than the thresholdvalue.